This invention relates to an adjustless V-belt and a method of manufacturing the same. More particularly, the term adjustless V-belt means a belt which automatically absorbs and adjusts its elongation which may be caused during operation, to maintain the tension constant. This invention also relates to a method of manufacturing the V-belt.
Wrapping connector driving belts such as flat belts, V-belts, and poly-V-belts serve, in general, to transmit power through the frictional force thereof. Accordingly, the belt requires tension predetermined according to the driving conditions. If the belt is elongated and the tension is decreased during use, then the force of the belt gripping the pulley is decreased. As a result, the belt slips. If the belt slips in this manner, then heat is generated in the belt, and the belt is further elongated while the degree of slip of the belt is further increased. Finally, the belt may fail prematurely by the heat generated therein. Accordingly, in order to improve the durability of the belt, it is necessary to provide a belt which is not significantly elongated and can maintain a tension higher than a threshold value at which slip is caused.
Recently, a technical concept has been studied in which a rope having large thermal contraction stress, such as a synthetic fiber rope made of, for instance, polyester fibers, is used as the tensile member of a belt, so that, when heat is generated in the belt to elongate the latter, the tensile member reacts quickly with the generation of heat in the belt to contract the belt. This tends to suppress the elongation of the belt. During a series of belt manufacturing processes, before the belt molding process, the thermal elongation treatment of the rope tensile member is extensively carried out in order to reduce the elongation of the rope tensile member. However, since the degree of thermal elongation treatment for the synthetic fiber rope is increased, the thermal contraction stress is increased during vulcanization.
Accordingly, when the synthetic fiber rope spirally wound on a cylindrical drum or a metal mold through a rubber layer not yet vulcanized is subjected to vulcanization, then the rope tensile member is contracted by the contraction stress. As a consequence, it is dropped into the rubber layer, and the contraction stress is reduced. Thus, the resultant belt is high in elongation. At worst, the rope tensile member in the rubber layer is disturbed, and it is difficult to maintain the pitch line of the rope tensile member uniform.
The above-described drawbacks accompanying a conventional method of manufacturing a rubber V-belt or a V-belt with cogs, and secondary difficulties which are involved in countermeasures effected to eliminate the drawbacks will be described with reference to FIGS. 1 through 5. These figures all disclose prior art techniques.
As shown in FIG. 1, a few plys of rubberized convas 24 are wound around a cylindrical metal mold 21 or a metal mold (not shown) on the outer wall of which protrusions are formed. A compressive rubber sheet 22 and an adhesion rubber sheet 23a which are not vulcanized yet are laminated on the rubberized canvas layer 24. Then, a rope tensile member 26 made of polyester fibers having a high thermal contraction stress is wound spirally on the adhesion rubber sheet 23a. Thereafter, an adhesion rubber layer 23b not yet vulcanized and a few plys of rubberized canvas 25 are wound on the rope tensile member 26 in succession, to form an assembly. Then, a molded belt blank is obtained by externally pressurizing and heating the assembly. Thereafter, the molded belt blank is cut into a plurality of rings to provide V-belts.
In this method of manufacturing V-belts, the rope tensile member 26 is embedded in the adhesion rubber layers 23b and 23a as the latter flows. However, the amount of rubber flowing between the parts of the rope tensile member spirally wound is very small, and the degree of friction obtained by the flow of rubber is therefore small. Thus, it is difficult to activate the surface of the rope tensile member 26. Since various blending chemicals and softeners are mixed in the adhesion rubber layers 23b and 23a not yet vulcanized, chemicals lowering the adhesion property are actuated. Therefore, the surfaces of the adhesion rubber layers 23b and 23a have unsatisfactory adhesion properties.
Hence, in combination with the rope tensile member 26 having the inert surface, it becomes difficult to bond the rope tensile member to the adhesion rubber layers 23a and 23b. Furthermore, since the rope tensile member 26 having the thermal contraction characteristic is wound on the flexible rubber layer not yet vulcanized, the rope tensile member is contracted during vulcanization. As a result, it drops as indicated by the arrows (FIG. 1) in the adhesion rubber layer 23a and the compressive rubber layer 22 below the rope tensile member. Thus, as shown in FIG. 2, the arrangement of the parts of the rope tensile member 26, i.e., the pitch line thereof becomes irregular. Accordingly, tension is non-uniformly applied to the parts of the rope tensile member 26. This tends to cause the belt to be broken prematurely.
In order to eliminate the above-described difficulty where the tensile member drops into the rubber layer by the thermal contraction, a prior art method may be employed in which, as shown in FIG. 3, a reinforcing canvas 28 is provided below the rope tensile member 26 to prevent the rope tensile member from dropping in the rubber layer. In this case, the rope tensile member 26 embedded in the adhesion rubber layer 23 is dropped in the lower parts of the adhesion rubber layer 23, to be brought into contact with the reinforcing canvas 28. Therefore, the reinforcing canvas may separate the belt into layers.
Shown in FIG. 4 is a reversal molding method. An upper rubberized canvas 25, an adhesion rubber layer 23, a rope tensile member 26, a compressive rubber layer 22 and a lower rubbered canvas 24 are wound on a metal mold 21 in succession to form an assembly. A molded belt blank is formed by pressurizing and heating the assembly, and then the molded belt blank is cut into a plurality of rings, which are turned inside out to provide the desired belts. In this conventional method, it is difficult to prevent the rope tensile member from dropping into the rubber layer. That is, the rope tensile member 26 is caused to drop into the adhesion rubber layer 23 as indicated by the arrows. As a result, the rope tensile member 26 is brought into contact with the upper rubberized canvas 25 as shown in FIG. 5. Thus, also in this case, the above-described difficulties may result with respect to the surface where the tensile member is provided.
As is apparent from the above description, in the above-described various prior art methods, no rubber layer is provided between the tensile member and the reinforcing canvas or the upper rubberized canvas, i.e., the tensile member is in direct contact with the canvas. Therefore, the adhesion property of the rope tensile member is lowered. Thus, the tensile member is liable to peel off during operation of the belt. Thus, the conventional methods described above are still disadvantageous for a variety of reasons.
The method according to this invention comprises the steps of:
(1) winding a longitudinally stretchable cover canvas around a metal mold on the outer wall of which strip-like protrusions and grooves are alternately provided;
(2) spirally winding rope tensile members subjected to thermal elongation around the cover canvas, the rope tensile member having a large thermal contraction stress of at least 3.5 g/denier which is the difference between a thermal contraction stress at 100.degree. C. and that at room temperature;
(3) winding an adhesion rubber sheet on the rope tensile members;
(4) winding a compressive rubber sheet containing short fibers laterally arranged, around the adhesion rubber sheet, thereby to form a molded belt blank;
(5) placing a cylindrical mold in the inner wall of which a group of cogs are formed over the molded belt blank; and
(6) heating and pressurizing said molded belt blank to permit a part of the adhesion rubber sheet to flow to fill the grooves of the metal mold, and to permit a part of the compressive rubber sheet to flow to fill the grooves of said cylindrical mold, to form either two groups of cogs on the upper and lower surfaces of said molded belt blank, respectively, or a group of cogs covered with the cover canvas on the surface of the compressive rubber layer.